Table of contents:
Video: Blue Spots On The Potato. Reasons For Proper Storage. Photo
The hostess brought a packet of potatoes from the store, peeled them, looked into the pan, and there - for one dinner for the family, and that was not enough. Outside, the tubers did not seem to have any flaws, and on the cuts there were completely black and blue spots. And now half of the potatoes go to waste …
What are these spots?
Alas, everyone has had bruises in life, especially in childhood. In the encyclopedia, this phenomenon called "bruising" is interpreted as follows: "Hemorrhage into soft tissues under the influence of blow or pressure with a blunt object." We try not to be exposed to such an action. But the potatoes, while it reaches from the field to the pan, the bit happens repeatedly. They beat him during harvesting, pour him from baskets into dump trucks, from dump trucks into wagons, unload wagons with shovels, and trample potatoes with their boots. So she gets bruises and bruises. Yes, dark spots in tubers are nothing more than bruises - the result of destruction of living tissue.
A tuber is an overgrown tip of an underground shoot. In this way, potato plants store nutrients for future offspring. Tuber tissues are composed of juicy and very complex cells, which, in addition to being pantries for starch storage, perform all the functions inherent in a living cell. The tuber breathes, complex biochemical processes of enzymatic transformation of some substances into others take place in it!
Phenolic compounds are localized in the vacuoles of healthy intact cells. If the cell is destroyed, then the contents of the vacuoles spread out and irreversible oxidation of polyphenols with atmospheric oxygen occurs. Basically, tyrosine is oxidized, under the influence of the tyrosinase enzyme. This is where dark-colored compounds are formed - melanin pigments.
Dark spots from bruises in potatoes do not appear immediately, sometimes only after several weeks of storage. After all, the tuber was not cut or split, only individual cells were destroyed, and the oxidation of polyphenols in them proceeds slowly.
The tuber darkens even if it is cut and left in the air. This phenomenon is familiar to everyone. Every housewife knows that raw potatoes peeled for future use should be put in water. However, it will darken in water, if you do not cook it on time, and the faster, the more tyrosine is contained in the tubers.
What kind of potatoes are often bruised? It is noticed: the one that is friable, starchy: its cellular tissues are fragile and poorly resist impacts. The same happens with immature tubers.
Chilled potatoes are also sensitive to damage: in the cold, everything becomes fragile. That is why, cleaned up late, in cold weather, it so often grieves us with its dark core.
Agricultural experts know that an excess of nitrogen fertilizers is bad for potatoes: the tubers are very large, but they remain young for a long time and therefore are more damaged. A small excess of phosphorus and potassium fertilizers, on the contrary, accelerates ripening. Potassium ions also increase the elasticity of cell membranes.
In general, potash fertilizers play a special role in potato growing. The potassium content in the tops can even predict the quality of the future harvest. If the dry matter of the tops of this element is more than 0.5%, then we can confidently say that the tubers will not darken. With 0.4-0.5% potassium in the tops, dark spots in the tubers may well appear. Such potatoes must be harvested gently, carefully. If there is even less potassium in the tops, then the tubers of the future harvest will almost certainly darken. It is better not to store them, but to use them as soon as possible.
But, of course, the most reliable remedy for "bruises" is careful handling of potatoes, the same as with apples, tomatoes and other juicy fruits.
It can also happen this way: raw potatoes did not darken, but they boiled them and cooled them a little - and now they look completely unappetizing: dark spots have appeared, and some tubers have completely turned gray somehow …
Here is a completely different process, it is caused by the interaction of iron and chlorogenic acid. This acid in raw tubers is in a bound state, and at a temperature of 80 ° C it is released and reacts with iron oxides, which is always sufficient in tubers.
Citric acid, which is also found in potatoes, also reacts with iron oxides. But the compounds are thus colorless. The degree of darkening of the potato depends on the ratio of these two acids, and it is a varietal sign, and therefore breeders always test all new varieties for darkening after cooking. However, growing conditions here can change the picture. For example, potatoes from peatlands (which are poor in potassium and rich in nitrogen), regardless of the variety, almost always darken.
You can preserve the whiteness of potatoes by adding a few drops of citric acid during cooking. However, keep in mind that this will slightly change the taste and the potatoes will lose their delicious friability. You have to put up with this, if, say, for a salad you need potatoes to remain beautiful even when cold, and bitter experience suggests that they will certainly darken.
Another kind of similar trouble occurs with potatoes when they are fried at high temperatures. At home, in the kitchen, we know how to avoid this, and intuition is usually enough for us to achieve a golden color from fried potatoes. But in the food processing industry, darkening is a lot of hassle when making chips or crispy straws. The finished products are not only ugly, but also bitter. The reason is in the reaction between reducing sugars (fructose, glucose, maltose, xylose, mannose) and free amino acids.
There are few such sugars in mature, freshly harvested potatoes, from 0.25 to 0.80%. However, already 1% is enough to darken the chips or straws. Meanwhile, if the potatoes were harvested in cold weather or too chilled during storage, they can accumulate up to 12% of reducing sugars. That is why potatoes for such processing are very difficult to store: they need a temperature of 7-8 ° C, but in this warmth the tubers germinate, and you need to take care all the time so that this does not happen.
So, let's summarize. Most of all, the fault is caused by the imperfect technology of harvesting, transportation and storage of potatoes. Knowing the reasons is half the battle. It is not difficult to eliminate them in your garden, much more difficult - in the collective and state farm fields. But the main thing is possible.